1. Field of the Invention
The present invention relates in general to a ballasting and power factor compensation circuit for a fluorescent lamp using a piezoelectric element, and more particularly to a high-power ballast using a piezoelectric transformer adapted to generate high power of the 28-Watt class, thereby being applicable to general fluorescent lamps of the 28-Watt class.
2. Description of the Prior Art
Piezoelectric transformers, which utilize mechanical vibrations to conduct voltage transforming and power transferring functions, are mainly used in power supply circuits because they have advantages of easy miniaturization and no requirement of any magnetic shield, as compared to known coiled transformers.
Recently, the application of such piezoelectric transformers has been extended to inverters for cold cathode ray tubes and to ballasts for low-power fluorescent lamps.
Fluorescent lamps, which are a kind of discharge tubes, must use high voltage at an initial drive stage so as to emit thermions required for a discharge operation. Such a fluorescent lamp has negative resistance characteristics in that the voltage applied to the fluorescent lamp following the discharge operation is reduced in inverse proportion to an increase in the amount of current flowing through the fluorescent lamp.
Therefore, a ballast used for such a fluorescent lamp must serve to supply high voltage required for a turning on of the fluorescent lamp while controlling the amount of current flowing through the fluorescent lamp, following the turning on, thereby maintaining a desired brightness.
By virtue of such a function, the ballast has a direct influence on the efficiency and life of the fluorescent lamp to which the ballast is applied. For this reason, the ballast should be configured to transform an AC voltage of generally, 50 Hz to 60 Hz into a high frequency of 20 kHz to 100 kHz to be used as a power for the fluorescent lamp, in order to achieve an improvement in the efficiency of the fluorescent lamp.
FIG. 1 is a circuit diagram illustrating a conventional fluorescent lamp ballast using a piezoelectric element. Referring to FIG. 1, a ballast capacitor Cext is connected, in parallel, to a fluorescent lamp 11. A piezoelectric transformer 12 is coupled at its output terminal to the fluorescent lamp 11. The input terminal of the piezoelectric transformer 12 is coupled to a rectifying stage via transistors S1 and S2. For an improvement in power factor, clamping diodes D21 and D22 and a charge pump capacitor Cin are connected to the input terminal of the piezoelectric transformer 12.
The piezoelectric transformer 12 is a PbTiO3 or Pb(Zr,Ti)O3-based piezoelectric element used for low power of the maximum 18-Watt class. In the illustrated case, the charge pump capacitor Cin is provided in the driving circuit of the ballast for a power factor compensation.
The ballast capacitor Cext is adapted to control the voltage applied to the fluorescent lamp in such a fashion that a high voltage is initially applied whereas a low voltage is applied in an ON state of the fluorescent lamp resulting from the high voltage, taking into consideration the load characteristics of the fluorescent lamp exhibiting a high impedance (for example, several mega-ohms) prior to the turning state while exhibiting a relatively low impedance (for example, several kilo-ohms) in the ON state.
In order to obtain a maximum power of 18 Watts or more using the above mentioned configuration, it is necessary to use, for the piezoelectric transformer, piezoelectric elements connected together in parallel or piezoelectric elements having a laminated structure.
For this reason, conventional high-power ballast driving circuits involve an increase in the number of constituting elements used and a complexity in the manufacturing process used. As a result, there is a problem of an increase in manufacturing costs.
Furthermore, the charge pump capacitor Cin adapted for a power factor compensation involves a problem in that it cannot meet a desired standard because it generally provides an insufficient improvement in power factor.
In addition, the ballast capacitor Cext is essentially provided at conventional ballasts. For this reason, in designing a multi-lamp ballast, there may be problems, such as an increase in manufacturing costs and a complicated design, due to an increase in the number of ballast capacitors used.
Therefore, the present invention has been made in view of the above mentioned problems involved in the prior art, and an object of the invention is to provide a high-power ballast applicable to a fluorescent lamp of the 18-Watt or higher class and the maximum 28-Watt class without any increase in the number of constituting elements used or any complexity in the manufacturing process.
Another object of the invention is to provide a high-power ballast driving circuit capable of achieving turning on and turning off operations without using any ballast capacitor.
In accordance with the present invention, these objects are accomplished by providing a high-power ballast for a high-power fluorescent lamp adapted to supply a drive voltage to the fluorescent lamp, comprising: a rectifier circuit for rectifying a low-frequency AC voltage to convert it into a direct current DC voltage; a power factor compensation circuit for compensating a power factor for the voltage outputted from the rectifier circuit, thereby boosting the level of the output voltage, the power factor compensation circuit including an active type power factor driver; and an inverter circuit for converting the DC voltage, outputted from the power factor compensation circuit, into a desired high-frequency AC voltage, the inverter circuit including an inverter, a resonator circuit connected to the inverter, and a high-power piezoelectric transformer having an input terminal connected to the resonator circuit and an output terminal connected to the fluorescent lamp, the high-power piezoelectric transformer serving to apply, as a drive voltage, the high-frequency AC voltage to the fluorescent lamp.